The present invention relates to a pneumatic radial tire suitable for use as a high performance tire particularly aimed to run on circuits and being capable of improving initial response characteristics at the time of steering and of achieving cuts in running times by restricting deformation owing to torsion at bead portions thereof.
In a high performance tire aimed for high speed running such as running on circuits, a 1xe2x80x941 structure is often employed for the purpose of improving lateral rigidity and of securing cornering force wherein a carcass A is formed of an inner carcass ply A1 (so-called inner ply) that is fold up from inside to outside around a bead core B and an outer carcass ply A2 (so-called outer ply) that is overlapped with and connected to the fold-up portion A1a of the inner carcass ply A1 by being extending inwardly to the outside of the bead core B as schematically illustrated in FIG. 6.
Since the inner and outer carcass plies A1, A2 enwrap therein the bead core B and a bead apex rubber C to form a shell structure in such a 1xe2x80x941 structure, movements of side wall portions can be advantageously restricted to improve the lateral rigidity.
However, even such a tire of improved lateral rigidity is not enough to improve response delays of a vehicle when its steering wheel is initially operated, and is not enough to cut the running time to the extent expected.
The inventor supposed that deformation owing to torsion of the bead portions in an initial stage of steering is a large factor thereof. More particularly, while movements are transmitted at the time of steering from a wheel rim to the bead portions, from the bead portions to the side wall portion, and from the side wall portion to a tread portion in this order, it is assumed that occurrence of torsional deformation of a pulled portion of the bead portions or bead cores at the initial stage of steering, causes a delay in transmission of movements to the tires.
It is therefore an object of the present invention to provide a pneumatic radial tire capable of achieving cuts in running time by basically providing first and second bead reinforcing layers at bead portions of a tire having a carcass of 1xe2x80x941 structure to thereby restrict deformation owing to torsion in the bead portions and to improve initial response characteristics at the time of steering.